Migrate user files from old machine to new machine

Following files/dirs are required for traditional Linux user management:
* /etc/passwd - contains various pieces of information for each user account

* /etc/shadow - contains the encrypted password information for user's accounts and optional the password aging information.

* /etc/group - defines the groups to which users belong

* /etc/gshadow - group shadow file (contains the encrypted password for group)

* /var/spool/mail - Generally user emails are stored here.

* /home - All Users data is stored here.

Pre

1. mkdir /move

Backup user account

1. We only want to keep user account except account used by system, so we only backup account with UID LIMIT >= 500.

        awk -v LIMIT=500 -F: '($3>=LIMIT) && ($3!=65534)' /etc/passwd > /move/passwd

Backup Groups 
        awk -v LIMIT=500 -F: '($3>=LIMIT) && ($3!=65534)' /etc/group > /move/group

Backup User & Group password 

1. awk -v LIMIT=500 -F: '($3>=LIMIT) && ($3!=65534) {print $1}' /etc/passwd | tee - |egrep -f - /etc/shadow > /move/shadow
2. cp /etc/gshadow /move/gshadow

Backup /home & /var/spool/mail

1. tar -zcvpf /move/home.tar.gz /home
2. tar -zcvpf /move/mail.tar.gz /var/spool/mail

[src] http://www.cyberciti.biz/faq/howto-move-migrate-user-accounts-old-to-new-server/